So, I'm thinking a perfect storm scenario might get many more to die. I like the long incubation period idea.
I actually believe that most people are so used to the normal flow and ease of life that they can't imagine a global apocalypse.
Al, have you read Justin Cronin's vampire trilogy or John Ringo's zombie series?
https://en.wikipedia.org/wiki/The_Passage_(novel_series)
https://www.amazon.com/gp/product/B0131UOUOC?ref=series_rw_dp_labf
Cronin's starts with contagious bats, and the third volume continues a story arc with a character who lives for centuries. Meanwhile the apocalypse's survivors have fueled up at an oil refinery and gone to Australia on a tanker where they eventually (over a dozen generations) rebuild civilization. They mount an archaeological expedition to North America and rediscover the "girl who saved the world".
John Ringo writes that apocalyptic stories are very much part of the human psyche because today's life is so tame. He claims that less than 1% of today's population is killed in violence each year, and WWII peaked at about 5% of the global population, but in nomadic times the average tribe lost 20% of its members to injuries or disease.
Ringo's apocalypse starts with a terrorist spraying a flu virus in public restrooms. Incubation takes at least a week to turn the humans into zombies but contagion spreads on contact the entire time. Vaccination is the only survival. The plot doesn't care who spreads the virus (or even why) but the survivors manage to leave NYC in a sailboat (no gasoline required), where they go on to save thousands more people who are adrift on military vessels or cruise ships. It's a combination of horrific violence and human nature's inevitable collapse of society.
Ironically one of Ringo's main survivor demographics is U.S. Navy submariners who are not exposed to the virus but who endure years of enforced quarantine on their boats while awaiting creation/production of a vaccine. Submarines can only carry 120-day food supplies but luckily nuclear reactors can run for decades and an active sonar pulse can kill a lot of fish...
For an honorable mention, I'd also suggest J. D. Molle's "The Remaining" series.
https://www.amazon.com/The-Remaining-6-Book-Series/dp/B01BY7JOMW
It's more focused on politics and military tactics but the premise is straight out of prepper conspiracy theories. His zombies eventually go through a genetic mutation to produce a new humanoid race that's bigger, stronger, faster, meaner,-- and hungrier-- than homo sapiens.
I don't share any of the authors' apocalyptic worldviews but I enjoy watching Ringo & Molles craft their tales. Cronin tends to blather on.